p27Kip1 is an evolutionarily conserved protein that regulates both cell proliferation, through inhibitory interactions with cyclins and cyclin-dependent kinases, and cell movement, through its inhibitory interaction with RhoA. From our previous published studies and from our preliminary results it is clear that both of these functions play an important role in normal cellular and organismal physiology and that misregulation of p27 can be an important step in the acquisition of an aggressive malignant phenotype in human cancers. The specific work of this proposal will utilize genetic, biochemical and molecular methods to address three broad issues related to p27 regulation and function. First, we will use mouse knock-in models expressing "separation of function" alleles of p27 to directly address what specific roles the regulation of cell proliferation and cell movement by p27 play during development and in adult cells and tissues. Second, we will use new technologies to elucidate the different biochemical mechanisms that regulate p27 protein abundance at each phase of the cell cycle. We will use these insights to create new mouse genetic models that will address the importance of each of the cell cycle-specific p27 regulatory pathways in normal cells and tissues. Third, we will study the regulation and function of p27 in tumor cells. We will determine the mechanisms that cause downregulation of nuclear p27 in different tumor models using both molecular methods and our new genetic models of p27 misregulation. We will also test the hypothesis that misregulation of p27 in tumors causes the loss of normal coordination between cell division and cell movement, thereby contributing to the proliferative and invasive phenotype of aggressive cancers.